Propylene homopolymer is widely used in the processing fields of injection, extrusion, tape casting and biaxial stretching due to the tailoring structure thereof. However, the common polypropylene molecular chain is of linear structure, which is unlike amorphous polymers, such as polystyrene PS with a region having property similar to the rubber elasticity in a wide temperature range. Thus, polypropylene cannot be thermoformed in a wide temperature range. Meanwhile, the softening point of polypropylene is close to its melt point. When the temperature is higher than the melt point, the melt strength and melt viscosity of polypropylene will decrease rapidly, thus causing the following problems including uneven wall thickness of the products during thermoforming, edges curling and shrinkage that would easily appear during extrusion, coating and rolling, and foam collapse during extrusion foaming etc. Therefore, the use of polypropylene in the fields of thermoforming, foaming and blow molding is limited. As a result, development of polypropylene with high melt strength and good ductility is always an interest issue. Said high melt strength polypropylene (HMSPP) means that the melt can bear higher strength at tensile fracture. In general, as to the current propylene homopolymer with a melt flow rate (MFR) of about 2 g/10 min, the higher melt strength thereof is up to 0.8 to 1N.
The main factor that affects the polypropylene melt strength is the molecular structure of the polymer, which comprises the size of the molecular weight, the molecular weight distribution, whether the molecular chain comprises long branched chains or not, the length and distribution of the branched chains, and so on. Generally, the larger the molecular weight of polypropylene is, the higher the melt strength of polypropylene is. However, the larger the molecular weight of polypropylene is, the more unfavorable it is for post-processing forming performance of propylene polymer. Therefore, taking the actual application of the materials into account, it is desirable to enable polypropylene to have a wider molecular weight distribution. In addition, it is also important to enable the polymer to contain a fraction with very high molecular weight, which can obviously increase the melt strength of polypropylene. In order to obtain propylene polymer with the best performance, the ideal polymer product should comprise a small amount of polymer fraction with very high molecular weight, a certain amount of polymer fraction with relatively high molecular weight and a large amount of polymer fraction with low molecular weight.
The disclosed process for increasing the melt strength of polypropylene generally comprises the method of increasing the polypropylene molecular weight, improving the molecular weight distribution or introducing branched structures by polymerization process technology, or the method of blending polypropylene with other amorphous or low crystallinity resins and elastomers during the polymer molding process. Among others, adjusting the polymerization process technology is commonly used, which comprises preparing polypropylene with wide molecular weight distribution by using a plurality of reactors or obtaining polypropylene with long branched-chains by using metallocene catalyst and in-situ polymerization, thus enhancing the melt strength of the final polymer. The most commonly used method is to prepare polypropylene with wide molecular weight distribution by using a plurality of reactors connected to each other in series. Generally, polypropylene with wide molecular weight distribution (MWD) is obtained by in-series polymerization in different reactors that are suitable for producing polymers with different molecular weights with different hydrogen amounts and different copolymerization monomers being selected. For example, one of the reactors is suitable for producing polymer with higher molecular weight, while another is suitable for producing polymer with lower molecular weight.
For example, U.S. Pat. No. 6,875,826 and U.S. Pat. No. 7,365,136 both disclose a process for preparing propylene polymers with high melt strength and wide molecular weight distribution. In the process, the multi-stage (in two reactors) propylene homopolymerization or copolymerization is carried out in tubular loop-gas phase polymerization reactors connected to each other in series by selecting a Ziegler-Natta catalyst with lower hydrogen response, wherein said Ziegler-Natta catalyst is foremost featured by using a siloxane containing cycloalkyl, such as dicyclopentyl dimethyl silane, as the external electron donor. Through controlling the hydrogen concentration in each reactor, polypropylene of high molecular weight fraction, i.e., MFR<0.1 g/10 min, is produced in the first stage, with the weight content thereof in a range of 10 to 35%; polypropylene of low molecualr weight fraction, i.e., MFR>0.5 g/10 min, is produced in the second stage, with the weight content thereof in a range of 65 to 90%; and the MFR of the final polymer is in a range of 0.1 to 20 g/10 min. Finally it is obtained after reaction a linear propylene homopolymer with high melt strength and a wide molecular weight distribution (Mw/Mn>6).
As well known, as to propylene polymerization, the species of the external electron donor will generally influence on the stereoregularity and molecular weight distribution of polymer significantly. When the above process is used to prepare homopolypropylene with wide molecular weight distribution in a plurality of reactors, it is desirable that the molecular weight and stereoregularity of the high molecular weight fraction, which can determine the mechanical properties of polymer, are as high as possible, especially with a certain amount of very high molecular weight fraction, while the molecular weight of low molecular weight fraction, which can determine the extrusion properties of polymer, is as low as possible and also with a higher content. However, the composition and characteristics of the catalyst are unchanged in the two reactors of the above patents. As a result, the reaction response of the catalyst on the molecular weight regulator, i.e., hydrogen, is uniform in the two-stage polymerization, which has a certain limitation for controlling or adjusting the properties of polymer chains.
Specifically, when the external electron donor with lower hydrogen response is used in the catalyst system, the molecular weight of polymer can be higher in the first stage for producing higher molecular weight fraction. However, when in the second stage for producing the lower molecular weight fraction, very high hydrogen content is required to meet the actual requirement because of its insensitivity to hydrogen. If the external electron donor with higher hydrogen response is used in the catalyst system, although the hydrogen amount is little in the second stage for producing lower molecular weight fraction, the molecular weight cannot be high enough in the first stage for producing higher molecular weight fraction, thus reducing the mechanical properties of the final products.
Moreover, CN1241196A describes a polypropylene resin composition and the use thereof; wherein a two-step method is used to obtain the polypropylene resin composition with high melt tension. In the method, polypropylene with high molecular weight is prepared in the first stage without hydrogen, and polypropylene with low molecular weight is prepared in the presence of hydrogen in the second stage. One and the same external electron donor, such as dicyclopentyl dimethoxy silane, is used in the whole process. The prepared polypropylene comprises the high molecular weight fraction with a molecular weight higher than 1.5×106. However, it still cannot solve the problems associated with the foregoing patents.
In CN1156999A titled as “The dual donor catalyst system for olefin polymerization”, two different catalysts are used in different stages. Tetraethoxy silane is used as the external electron donor in the first stage, and dicyclopentyl dimethoxy silane is used as the external electron donor in the second stage. Both of CN1612901A and U.S. Pat. No. 6,686,433B1 also similarly disclose the teachings. The objects of these patents are not to obtain macromolecular so as to further obtain polypropylene with high melt strength. The process steps thereof similarly comprise: first preparing the smaller molecular polypropylene, then preparing the larger molecular polypropylene in the second stage, thus obtaining polyolefins with high crystallinity. If using said processes described in these patents to produce polypropylene, said external electron donor with a high hydrogen response, which is added in the first stage for preparing low molecular weight polymer, also works in the second stage, so that super molecular weight polymer cannot be prepared in the second stage. Similarly, the high melt strength propylene homopolymer with advantageous mechanical property and processing property cannot be obtained according to the above patents.
As to the actual applications of some polypropylene, such as foaming products, the melt flow rate (MFR) thereof is required to be about 2 to 3 g/10 min. Because of limitation in the above polymerization processes, the distribution of the three fractions, i.e., the very high molecular weight fraction, the high molecular weight fraction and the low molecular weight fraction, in the polymers is not satisfactory. Thus, the properties of the final polymer are harmed to some extent.